1. Field of the Invention
The invention relates to the field of telecommunications. More particularly the invention relates to the field of multiplexed communications. In still greater particularity, the invention relates to a method of removing harms to telephony and other services delivered through bus-based hybrid fiber coaxial cable (HFC) networks. By way of further characterization, but not by way of limitation thereto, the invention uses detection and automatic disconnection of spurious (jamming) signals that may emanate from one or more end-user customer locations connected to an HFC network.
2. Description of the Prior Art
Information, and access to it, has received significant attention recently. The building of an "information highway" compared to the national interstate highway system begun in the 1950s has been made a national priority. There are currently three wireline transport elements available for such a highway: (1) fiber optic cable; (2) coaxial cable; and (3) twisted copper pair cable ("twisted pair"). Presently, twisted pair cable predominates, certainly in the local loop portion of telephone networks. Coaxial cable has been used widely by cable television companies. Both telephone companies and cable companies have made use of fiber optics for main or trunk line signal transport.
Fiber optic cable can carry more information over a greater distance than coaxial cable, while coaxial cable can carry more information over a greater distance than twisted pairs. Because twisted pair is the predominant local loop technology, at least in the telephone industry, attempts have been made to develop technologies which will increase the carrying capacity of copper. In reality, copper wire is a very efficient transport means for traditional telephony services.
Within the telephony industry, the term "broadband" denotes a very high digital line rate, such as the 156 Megabits per second (Mb)s) optical line rate of new SONET OC3-level fiber optic systems. The term "baseband" describes the original (unmodulated) form of the electrical or optical signal associated with a single service that is typically presented to the network by a subscriber, and the final form of that signal presented from the network to a subscriber. The baseband signal can be either analog or digital in form, and is further characterized as the direct electromagnetic representation of the base information to be transmitted, with no other carrier or subcarrier energy present. A baseband signal may be carried directly on a transmission line, such as a twisted pair of insulated copper wires or an optical fiber. A baseband signal may also be used to modulate a carrier signal for transmission on a variety of transmission systems (e.g., radio). In telecommunications, the term "passband" describes the range of frequency spectrum which can be passed at low transmission loss through a linear transmission system. Modulated carrier signals presented to such a system will be delivered in their original form with minimal loss and distortion, as long as such signals fall within the absolute limits of the passband range of frequencies and the dynamic range of signal amplitude for a given linear transmission system.
An example should help clarify the relationship between baseband and passband. The electrical signal that is present at a telephone jack during a conversation is the baseband electrical signal representation of the talker's voice. This baseband signal is typically transported to the telephony switching office by a twisted pair of insulated copper wires. At the central office, the signal goes through the switch and is typically converted to digital form and multiplexed in the time domain for transmission through baseband digital transmission systems that carry such signals on copper or fiber optic cables to other locations. The baseband digital transmission system may carry thousands of individual telephone calls on the same transmission line. Even though there are multiple calls in progress on the same transmission line, such a system is still defined as "baseband" because there is no modulation of a carrier or subcarrier signal anywhere in the system, and, at any given instant of time, there is only a single subscriber's signal actually present at a given point on the line. As the original talker's signal reaches the other switching office involved on the call, it is converted back to the original analog form and put on the copper pair connected to the far-end telephone set, once again in baseband form.
Passband techniques can also be used to provide telephony services. In cable television systems configured for telephony services, the baseband analog telephone signal is used to modulate a carrier signal. The modulated carrier signal can be assigned a particular frequency within the passband of the linear transmission system. A number of such modulated carrier signals, each assigned a different carrier frequency in the passband, can be transmitted all at the same time without mutual interference. At the far end, a selected modulated carrier signal must be demodulated to remove the carrier signal and recover the baseband signal associated with the service. If the linear transmission system is operating properly, the derived signal will be delivered to the far-end telephone set, once again in baseband form.
Fiber optic-based systems are preferable to copper-based networks because of their high bit rate transport capability. Information services that require true broadband rates require fiber or coaxial cable technology, as a practical matter. Even low-end (i.e., POTS "plain old telephone service") services will reflect a lower per-subscriber cost on fiber, compared to present copper-based delivery systems. Specifically, fiber-based systems that provide residence telephony to groups of 4-8 subscribers with fiber to the curb (FTTC) are expected to achieve cost parity with copper in the near future. However, the cost to replace the existing copper plant in the U.S. with fiber optics is estimated at hundreds of billions of dollars. Thus the length of time required to achieve this conversion could be decades.
One possible alternative to fiber or copper networks is a hybrid network which utilizes existing facilities and employs fiber optics, coaxial cable and copper wiring. Such a network would allow the delivery of many advanced services and yet be more cost efficient to allow earlier conversion to a broadband network with significant fiber optic capability included.
In general, hybrid networks combine a telephony network and a video network. One drawback of such a network is some duplication of equipment required to transport the separate signals. That is, if, for example, the telephony services could be sent over the video network, then a substantial portion of the cost and complexity of the hybrid network could be eliminated. However, in order to send telephony and video signals over the same transport medium, the unique characteristics of each signal must be addressed. For video signals this is not as difficult as some of the issues surrounding transport of telephony signals. That is, video signals are generally sent in one direction, from the provider to the subscriber, while telephony requires two-way transport. As video evolves into interactive video, however, two-way video signal transport issues will also become significant. When telephony or interactive video signals are carried on a bus-based network there exists the potential for the introduction of spurious signal (jamming) energy which would harm the network.
The Federal Communications Commission (FCC) has codified regulations to address the issue of harms to the network. Part 68 of the Code of Federal Regulations (CFR) defines harms to the telephone network, to third parties, and to billing systems, for connections between networks and non-carrier end-user (NCEU) customer premises equipment (CPE). Specific technical parameters such as signal power, longitudinal balance, and encoded analog content are defined, including numeric limits for each parameter. The fundamental purpose of CFR Part 68 is to set safe technical operating limits for CPE connected to telephone networks in order to prevent defined harms. Part 68 also prescribes a CPE registration process that certifies equipment manufactured for such applications. CPE must be certified and assigned an FCC Registration Number, in order to be legal for connection to the network.
Traditional loop plant telephony architecture provides each NCEU a dedicated transmission path all the way back to the telephone switching central office. The physical/logical "star" topology (FIG. 1A) of typical telephone networks and the CPE technical limits set forth in Part 68 work together to prevent harms. As an example, registered CPE conforming to Part 68 longitudinal balance and signal power requirements will maintain pair-to-pair isolation in the twisted copper pair loop plant, and no crosstalk (interference) harm will be experienced by third parties connected to the same network. If, however, the CPE unbalances the otherwise balanced network twisted pair, and/or exceeds signal power limits, the neighbors of the offending NCEU CPE may experience service disruptions.
Part 68 can be described as technology-specific regulation. Simply stated, Part 68 does not contemplate the bus-based HFC architecture (FIGS. 2A and 2B) as a delivery vehicle for telephony and other interactive services, as part of the public switched telephone network (PSTN). New technology ("cable phone") provides the ability to derive telephony and other interactive services on HFC networks, such as may be employed by cable television system ("CATV") operators. HFC networks are susceptible to inadvertent or malicious jamming, due to the common bus topology that provides all connected parties with direct electrical access to all of the downstream and upstream bandwidth on the system. The jamming signals do not even have to be directly on the frequency of a particular service to produce harm. Jamming signals may take many forms, including single frequencies, harmonics, intermodulation products, and combinations thereof. Interfering jamming signals in a portion of the system frequency spectrum that has no services may still harm services in other portions of the spectrum.
Existing cable phone products can be found as part of the network either external to customer premises, or if within premises, in a form similar to the traditional set-top box associated with video services. In either case, the fundamental HFC bus architecture provides each connected NCEU with direct electrical access to the entire upstream and downstream bandwidth at traditional F-fitting coaxial cable jacks inside premises. Conventional telephony star architectures do not permit such direct access. Absent some means to prevent the potential harm of jamming from NCEU premises, HFC technology for loop connection to the PSTN puts lifeline telephone services of all customers so served at substantially greater risk. State-of-the-art HFC systems currently connect 300-400 homes to a common coaxial cable bus. One interfering jammer has the ability to deny service to all connected homes. While disruption of broadcast video, or other "one way" video services offered as part of conventional cable service, would be an aggravation, denial of lifeline telephone service to an entire area of a community by simple jamming is completely unacceptable.